Interactive effects of aluminium and phosphorus on microbial leaf litter processing in acidified streams: A microcosm approach Hugues Clivot a, * ,1 , Faustine Charmasson a , Vincent Felten a , Jean-Pierre Boudot b , François Guérold a , Michael Danger a a Université de Lorraine, CNRS UMR 7360, Laboratoire Interdisciplinaire des Environnements Continentaux (LIEC), Campus Bridoux, Rue du Général Delestraint, 57070 Metz, France b Université de Lorraine, CNRS UMR 7360, Laboratoire Interdisciplinaire des Environnements Continentaux (LIEC), Faculté des Sciences, 54506 Vandoeuvre-lès-Nancy, France article info Article history: Received 17 August 2013 Received in revised form 20 November 2013 Accepted 27 November 2013 Keywords: Acidification Leaf litter decomposition Aluminium Phosphorus Aquatic hyphomycetes abstract Decline in pH, elevated aluminium (Al) concentrations, and base cations depletion often covary in acidified headwater streams. These parameters are considered as the main factors reducing microbial activities involved in detritus processing, but their individual and interactive effects are still unclear. In addition to its direct toxicity, Al can also reduce the bioavailability of phosphorus (P) in ecosystems through the formation of stable chemical complexes. A three week microcosm experiment was carried out in acid conditions to assess the interactive effects of Al (three levels: 0, 200, and 1000 mgL 1 ) and P (25, 100, and 1000 mgL 1 ) on alder leaf litter processing by an aquatic hyphomycete consortium. Our results showed that Al alone reduced fungal growth and altered fungal decomposer activities. High P levels, probably through an alleviation of Al-induced P limitation and a reduction of Al toxic forms, suppressed the negative effects of Al on detritus decomposition. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Leaf litter breakdown is a key process in woodland streams, where primary production is notably limited by the riparian canopy and the oligotrophic status of these streams (Vannote et al., 1980). Indeed, in such heterotrophic ecosystems, allochthonous litter in- puts represent the major source of energy and nutrients for all the detritus-based food web (Fisher and Likens, 1972; Webster and Meyer, 1997). Various studies reported that this ecosystem pro- cess is severely impaired in headwater streams subjected to anthropogenic acidification (Chamier, 1987; Mulholland et al., 1987; Meegan et al., 1996; Dangles et al., 2004), mainly through its negative impacts on acid-sensitive key shredders (Dangles and Guérold, 1998, 2001; Simon et al., 2009) and on aquatic hypho- mycetes involved in leaf litter processing (Chamier, 1987; Baudoin et al., 2008). In particular, the latter are considered as the main mediators of microbial leaf litter conditioning (Gessner and Chauvet, 1994), which is an essential step before leaf litter con- sumption by stream detritivores (Bärlocher and Kendrick, 1981; Arsuffi and Suberkropp, 1988). For instance, several factors have been identified as potentially responsible for reduced activities of microbial decomposers in acidified streams. Acidic pH and low calcium (Ca) concentrations can notably reduce the efficiency of pectin lyase activity, which is involved in leaf litter maceration (Chamier and Dixon, 1982; Jenkins and Suberkropp, 1995). Other studies reported that aluminium (Al) mobilized from soils to surface waters as a result of acid depositions can affect microbial communities. Indeed, under acidic conditions, monomeric forms of Al are known to be toxic for most aquatic organisms (Gensemer and Playle, 1999) including microorganisms (Piña and Cervantes, 1996). Increasing Al concentrations can decrease microbial respiration on decaying leaves (Dangles et al., 2004) and alter growth and conidial production of aquatic hypho- mycetes (Chamier and Tipping, 1997). In addition, recent studies on microbial enzyme activities suggested that acidification and, in particular, elevated Al concentrations in streams could induce a phosphorus (P) limitation for microbial decomposers that could, in turn, constrain leaf litter processing (Simon et al., 2009; Clivot et al., 2013b). Indeed, Al could interfere with the P cycle in acidified waters through abiotic interactions (Kopácek et al., 2000) and in- hibition of extracellular phosphatases (Bittl et al., 2001), thus * Corresponding author. E-mail addresses: hugues.clivot@univ-lorraine.fr, clivot.hugues@hotmail.fr (H. Clivot). 1 Present address: Laboratoire Agronomie et Environnement, Université de Lor- raine e INRAUMR1121, 2 Avenue de la forêt de Haye, 54500 Vandoeuvre lès Nancy, France. Contents lists available at ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol 0269-7491/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.envpol.2013.11.024 Environmental Pollution 186 (2014) 67e74